An ABS resin and a high impact polystyrene resin, in each of which a rubber component is blended in a resin composition, have been provided as an impact resistant resin. Since a diene polymer used as a rubber component which imparts the impact resistance has many chemically unstable double bonds in its main chain, the resins are liable to be degraded by ultraviolet rays or the like, and are inferior in weather resistance generally.
An ASA resin has been proposed as a resin that has better weather resistance than an ABS resin. An ASA resin can be obtained by graft copolymerization between an acrylonitrile compound and a styrene compound in the presence of an acrylic rubber having no double bonds. Since acrylic rubber is contained in ASA resin, an ASA resin is excellent in weather resistance but has shortcomings of low impact resistance.
A method for increasing a swelling ratio of acrylic rubber has been conducted in some cases for improving an impact resistance of an ASA resin. In this case, a surface glossiness of a resin molded article is seriously degraded. When an impact resistance is improved by increasing molecular weight of a resin, excellent moldability which is the feature of an ASA resin is degraded.
In order to obtain a high level of balance with the weather resistance and furthermore with the surface smoothness and the moldability, Japanese Patent Publication H3-66329B has proposed a specific ASA resin containing rubber obtained by compounding a small amount of conjugated diene rubber and a large amount of acrylic acid ester rubber. When continuous molding is performed using this specific ASA resin, a gas generated during molding is deposited on a mold to form an oily material, and a deposit thereof is transferred to a molded article side, so that an appearance thereof is degraded. Hence, the oily deposit adhered to the mold is required to be periodically removed by cleaning, and hence continuous moldability becomes inferior. Even when an amount of the gas thus generated is small, appearance defects, such as a flow mark and a silver streak, are generated.
Patent Literature 1: Japanese Patent Publication H3-66329B